Scooterotica

set up is as follows;
suzuki 195, self tuned, quite racy timings.
phbh 28mm, 52 pilot (bit too rich) 40 slide, 300 float jet, av266 with x7 2nd clip (runs great liek this, drop it a clip and its too rich, hard to rev out) and main jet might be 132? definitely around 130 mark. still fine tuning the jetting.
running through mb panel filter.
exhaust jl3
ignition indian, with reedspeed cdi set at 25 just after tickover retarding at 4k (I THINK) to 16/17 degrees (need to double check timing after changing stator, either way its in its most retarded position so should be about right... famous last words!)

heres the problem. the bike has a proper power band. I have noticed when i really push it and then de throttle to change gear, the revs kind of boomerang, very odd sound, doesnt sound right and to me sounds like things getting a bit tight or a bit lean. I am still running in but done nearly 300 miles on ally barrel.

took it out 2 up on fast 3 lane dual carriage way and revved it out in 4th, according to my mates honda speedo we touched 75, so realistically probably got up to 70. went to ease back on the throttle to attain a cruising speed, probably down to quarter throttle, and funny rattling sound just at that throttle position. never heardd pinking before but thats got to be it. if revved out it went away. also i pulled the clutch every so often to rev it to cool it a bit, and the revs hung and when revved made that familiar constricted sound.

Round town its sort of fine jetting wise, but this high speed running concerns me.

Im going to try a x13 needle, but seeing as 3rd clip on x7 is too rich, logically to me that should make it way too rich.

i cant really up the pilot anymore. only thing i can think of is a 30 slide. but to me that doesnt explain the strange throttling off symptoms when in the power, its fine if i dont rev it too hard, but if i really get through the revs it does it.

a part of me feels like it could be the top of the bore is still tight. when i had it rebored, patch at scooter surgery told me the bore was oval and he struggled to get the top clearance as big as bottom. i know that sounds odd but i trust him, but i havent been shy to rev it out and run the top part in.

any help appreciated!

What heads on it?

it was a sr175 barrel and head. i had the head re worked by harry barlow to suit my piston. he did the chamber too, but i dont know the actual volume. I am unnder the impression that harry was probably on the conservative side anyway, so im not wholly convinced its a compression issue, but i suppose until its measured there's no way of truly knowing...

ps, im not suggesting harry barlow is a tory.

i would deffo be checking the head volume on this firstly and re setting the ign timing to suit ,sounds to me like you would be better off with basic static timing tbh ,or have it set at say 20 coming down to 12 depending on what sort of "racey "self tuned port timings you have ?

well i say race timings... theyre not crazy really just it has a powerband, which im finding hard to jet in and also make day to day town riding not impossible.
timings are from memory, around 186 exh 129 tr and 165 inlet? im not 100% with the accuracy of that, but either way its worked very nicely.
i may drop the timing slightly or try it fixed as with the readspeed i can change it on the fly. would be a shame to loose tractability low down as even though it has a sharp kick in pants band it pulls 2 up nicely round town.
will investigate that first and maybe try x13 needle as some have reported lean spots with that combo.
cheers all

I think the Honda speedo may be optimistic as well. 70/75 two up and running in ?

I read a lot of maybes and possiblys but have you accurately measured TDC and marked it clearly and then set timing using a timing disc and strobe so that you can see if the ignition retards early enough?
Symptoms sound "weak". Checked for air leaks? Using best petrol?
Check it's not something else rattling like toolbox door or loose exhaust baffle.
3 elements: Fuel; compression; ignition. Eliminate doubt one at a time.

cheers chris.
I know what you mean about many maybes etc, and really its overcomplicating the problem.
Yes i did strobe the timing up properly, but need to check that the unit is retarding properly since fitting a new stator.
I will do a leakdown test at some point, but im pretty confident theres no huge leaks as i had to rebuild the top end not long ago for that very reason.
I agree the speedo was ambitious. i may be fitting a sip speedo soon, then i can show off down the pub lol.

I dont have the luxury of having run this setup for very long, so Im still not convinced i have the jetting spot on. I will recheck the timing too, maybe knock it back a touch, and then if i still have issue ill check out compression.

thanks for everyones input!